Project Summary Widespread use of combination antiretroviral therapy (cART) has led to increased survival of Human Immunodeficiency Virus (HIV)-infected patients. As a result, chronic diseases are increasingly replacing acute infections as important causes of morbidity and mortality. One of these chronic diseases is atherothrombosis, the underlying mechanisms of which are not fully elucidated. Here, we demonstrate that monocytes and platelets from HIV-infected, cART-treated individuals exhibit reduced levels of the host restriction factor Tetherin (BST-2/CD317), and that Tetherin loss is induced by exposure of healthy cells to the viral protein Tat, and to other inflammatory mediators of cellular origin, via proteasome-dependent mechanisms. We also show that over-expression of degradation-resistant Tetherin, which contains a S3T4S5 substitution mutation, in monocytes causes sequestration of fully formed microparticles (MPs) on the cell surface. These findings will be leveraged to test the overall hypothesis that HIV promotes thrombosis via Tetherin-dependent release of cellular MPs. Briefly, we will examine the loss of Tetherin and associated release of MPs by employing in vitro models of HIV infection (Aim 1), novel mouse models of HIV-associated thrombosis in vivo (Aim 2), and prospectively determine how these events are linked in HIV patients receiving cART (Aim 3). These studies will bring together four established investigators and two emerging investigators with proven expertise in virology, platelet/monocyte biology, MPs, thrombosis, and HIV clinical research. Importantly, proposed studies will leverage research infrastructure offered by the District of Columbia Center for AIDS Research (DC-CFAR). Our studies contain great potential for clinical translation. Changes in Tetherin expression will be directly tested in human subjects and linked with atherothrombosis. As such, revealing the mechanisms through which HIV and cART regulate the production of microparticles in monocytes and platelets will provide a critical basic science foundation so as to understand how these molecular pathways function in mammalian cells, while providing insight into how HIV triggers atherothrombotic events in patients who are or are not receiving cART. In summary, our proposal is discovery-oriented, translational, and is responsive to the goals of RFA (RFA-HL-14- 024).